VDG ROLLERS AND COMBS (c)1996 William J. Beaty
(See also amasci.com/emotor/vdg.html)
ROLLER MATERIAL
If your VDG only contains a motor, and it does NOT use a separate DC high
voltage supply, then the surfaces of the two rollers must be composed of
two different materials, and the materials must not be the same as the
belt material. The classic VDG machine uses a rubber belt, a metal
roller, and a plastic roller.
Polyethelene seems to work OK as the plastic roller when it is used with a
rubber belt. If a metal roller is too hard to make, build a wood or
plastic one, but coat it with aluminum foil attached with contact cement.
Only the thin surface layer of the plastic roller is important, so it's
possible to make a roller out of one sort of plastic, and then coat it
with another type. Then as far as the comb-charging effect is concerned,
the VDG machine will "think" that the rest of your roller is made of the
same material as the coating. For example, you could make a nonconductive
wood or polyethelene roller, then wind it with various sorts of plastic
adhesive tape or paint it with various sorts of plastic-based paints or
glues in order to find a material which works best. I haven't tried this
myself yet, so I can't tell you if one sort of tape makes a better roller
surface than just polyethelene. Try it and see. It is possible that
you'll discover a material which gives a higher generator voltage, or
works better in high humidity than just plain polyethelene. Note: I'm not
sure that a plastic-coated METAL roller will function properly, the metal
will act as a capacitor plate and cause the e-field of the charged plastic
to concentrate below the plastic rather than out at the charging comb.
Another thing I don't have much direct experience with: *plastic* belts
with rubber rollers. Since rubber belts are usually hard to obtain, while
things like garbage bags (cut into rings) should work well, it makes more
sense to use a plastic belt with a rubber roller. And surplus sources
often have parts of photocopiers and laser printers which incorporate long
rubber rollers having an aluminum core. Saw the roller to length as
needed? And if you're building a VDG from scratch, why not design yours
with a vertical length which allows easy use of belts snipped as rings
from unmodified lawn-leaf garbage bags!
ROLLER SUGGESTIONS
- Polyethelene cylinders from plastic supply company
- Plastic water/sewer pipe with end caps
- Large wood rods
- Stack of wood disks
- Metal or plastic turned in a lathe
- Coat with alum. foil or nickel paint to make "metal" rollers
More about electrostatic materials:
The Triboelectric Series
http://amasci.com/emotor/tribo.txt
In the above series, choose two materials which are far apart in the list.
ROLLER SHAPE
How can we keep the belt from wandering and falling off the rollers?
First, the two rollers must be adjusted so they are nearly parallel and
the belt only drifts sideways very slowly. The slow drift can then be
stopped with a secret: make the rollers barrel-shaped. Belts tend to
climb up over barriers on the rollers, but they will avoid dropoffs. The
roller need not be smoothly curved, just give the ends a big bevel like
so: _______________
/ \
/ \
___| |___
| |
\ /
\_________________/
You might also experiment with putting a lump in the center of your flat
roller, for instance by winding it with many turns of electrical tape (or
whatever material you've chosen for a coating.) I haven't experimented
with this myself, but I think it should work.
THE COMBS
For the combs I've tried many different things. I've soldered short wire
stubs to the side of a thick wire. I've tried soldering needles on
instead, but it's hard to solder to steel without the proper solder-flux.
I've used pieces of stranded wire: strip off a couple inches of plastic
insulation, spread out the wire, then snip it off flat. I've used window
screen. Just cut a square of window screen and you end up with needles
(but fold over the cut edges, since sometimes the unwanted needles do bad
things.)
Usually the combs aren't critical. All of the above techniques seem to
work fine. It's much more important to get the surfaces of rollers and
belts right... and to make them totally clean and oil-free.
DOUBLING THE VDG OUTPUT CURRENT
A simple VDG will put imbalanced charge on the belt, transport it to the
other end of the generator, then route it from the belt through the comb
to the output terminal. However, this only uses one half of the belt.
Why not have both ends of the VDG place opposite charges on opposite ends
of the belt? This will double the output current of the generator, which
will make it perform better in high humidity, cause spark repetition time
to be twice as fast, and may even raise the maximum output voltage.
There is a simple way to accomplish this. If one roller is plastic, make
the other VDG roller from metal, and totally insulate all parts of it from
the rest of the VDG. In this way the roller will pick up some imbalanced
charge from the belt, become charged to the same polarity as the belt, and
will force its adjacent comb to put out twice the normal current. Rather
than partially neutralizing the belt charge, the adjacent comb will charge
the belt with entirely the opposite polarity, and this opposite charge-
imbalance will take the return path to the other end of the generator.
For example, suppose there is a plastic roller in the base of the
generator, and an insulated metal roller within the upper sphere. Frank
Lee's large VDG kit (see SCIENCE FIRST INC.) employs this method. Assume
that the lower roller and comb gives the upwards-moving belt a positive
polarity. The inside of the upper hollow sphere behaves as if it is
grounded, so normally the upper comb will simply act to provide a
conductive path between the belt and the sphere. The positive charge
flows from the belt, to the upper comb, and out to the metal sphere. As a
result, the belt surface is almost neutralized. But if the upper roller
is not neutral, if instead it is positive, then it will make the comb act
negative, and will drive extra charge-flow to the comb. Once the belt has
passed by the upper comb, the belt surface will be more than just
neutralized, it will now have a negative charge imbalance. If the belt
transports positive charge upwards AND negative charge downwards, then
total current is doubled.
The upper metal roller needs to have the same polarity as the incoming
belt. It may pick up some charge from the belt on its own. However, this
effect can be strengthened if the metal roller is given its own small comb
upstream from the main comb. This extra comb will extract charge from the
belt and guarantee that the roller becomes electrified. Simple contact
between metal roller and belt may not supply enough charge to the roller
to be enough to conquer any surface leakage between the roller shaft and
other parts of the generator. A secondary comb makes sure the roller has
a strong charge. This small comb must be connected electrically to the
metal roller. The metal roller and its small comb must be "floating",
they must have no conductive connections to any other metal structures.
In the Frank Lee VDG kit, this small comb is suspended below the metal
roller within the space created by the two parts of the belt. The small
comb extracts charge from the inner surface of the belt as the belt makes
its way upwards.